Refractometers of this type are also frequently referred to as autorefractors and serve for measuring the refraction of an eye and, if applicable, the ametropia thereof from a distance.
The basic principle of such refractometers is described in U.S. Pat. No. 4,761,070. A light pattern is produced with the aid of an optical projection device, for example, light-emitting diodes, and projected on the retina of the eye. The projection of the light pattern is realized such that the light pattern is focused on the retina. The light pattern reflected on the retina is viewed through the eye lens by means of an optical viewing device that comprises a photoelectric sensor, for example, a video camera, such that an image pattern is projected on the photoelectric sensor. This image pattern is recorded with the photoelectric sensor and evaluated with an evaluation device, preferably digital image data processing software. The light pattern projected on the retina is characteristically distorted in accordance with the refraction properties of the eye such that the refraction properties of the eye can be derived by evaluating the degree of distortion.
One important aspect for the correct derivation of the refraction properties is that the eye is arranged a certain distance from the refractometer. Deviations in the distance between the eye and the refractometer result in a correspondingly altered distortion of the light pattern on the photoelectric sensor such that these deviations lead to measuring errors in the determination of the refraction properties.
In order to prevent these measuring errors, DE 101 53 397 A1 describes a refractometer system that also features a measuring unit for determining the distance between the device and the patient. The data obtained from the distance measurement can be used for positioning the patient correctly in front of the device. It would therefore also be conceivable to incorporate the distance information into the evaluation and thusly correct the refraction measurement data accordingly.
Suitable measuring systems for determining this distance are described in DE 101 53 397 A1 and respectively consist of an ultrasonic transceiver and an optical distance measuring system, in which a light pattern is projected on the forehead of the person being examined in order to measure the distance between the forehead and the measuring system.
The disadvantage of the measuring device described in DE 101 53 397 A1 is that only the distance between the head of the patient and the refractometer can be measured. This measurement is therefore inaccurate and, in principle, does not suffice for suitably correcting the refractometer data because the position of the head does not contain definitive information on the position of the eye. The correction of the measuring data consequently leads to unsatisfactory results.